Not applicable.
1. Field of the Invention
The invention concerns a drilling riser for use at great sea depths for oil drilling, and the application of this for controlling the riser margin, i.e. the overpressure being necessary to maintain in the drilling riser in order for a voluntary or nonvoluntary disconnection of the drilling riser not to lead to a blowout of gas, oil, formation liquids or other fluids.
Norwegian authorities, represented by The Norwegian Petroleum Directorate (NPD) normally require two independent pressure barriers during all drilling or well operations. However only one barrier is required, namely the drilling mud, until the surface casing, usually 20xe2x80x3, is installed. When the operator""s drilling program is evaluated, the requirement for a riser margin is carried out with the demand for two barriers. The riser margin is to be understood as:
xe2x80x9cHydrostatically exceeding pressure provided by an increased mud weight in order to compensate for the loss of hydrostatic pressure in the case of a sudden replacement of the mud column in the marine drilling riser with sea water (up to the sea surface level).xe2x80x9d
The limitation for the riser margin is the pore pressure and the fracturing pressure of the rocks at the lower end of conductor pipes or the casings. If the mud pressure is higher than the pore pressure there is a risk of leaking of drilling mud into the geological formation, and lost circulation with a resulting risk of an uncontrolled blowout. If the riser margin is too low, a risk is present that the pore pressure in the rocks exceeds the hydrostatic overpressure in the mud after the drilling riser has been disconnected from the wellhead, a situation which also can result in an uncontrolled blowout.
Drilling liquid is used during oil well drilling for several reasons. The drilling liquid lubricates the drillstring so that power is not unnecessarily lost to an unwanted degree against the borehole wall, the borehole casing wall and the drilling riser. The drilling fluid also has high heat capacity and transports away heat which arises by friction during drilling, both from the drill bit, the borehole""s bottom and wall, and also by friction arising between the drillstring and the casing and the drilling riser. The drilling liquid is circulated by pumping it down through the drill string, out through nozzles in the drillbit, and back up (out) again between the drill string""s outside and the borehole wall, and further on the inside of the casing and through the blowout preventer and up into the drilling riser. The drilling riser comprises the connection between the blowout preventer on the seabed and the drilling vessel or drilling platform which (usually) floats at the sea surface. On the drilling vessel the drilling liquid is treated by filtering of cuttings and sand, and the density and the chemical composition is checked and adjusted before it is pumped down again into the borehole. The column of drilling fluid exerts a pressure p towards the borehole wall in every point according to the formula
p=p0+xcfx81m1ghmxe2x80x83xe2x80x83(1)
where xcfx81m1 is the density of the drilling liquid, g is the gravity acceleration and hm is the depth of drilling liquid below the surface of the drilling liquid. p0 is an extra or optional static overpressure exerted on the drilling liquid at the surface, usually the atmospheric pressure.
The normal situation is to let the drilling liquid return out near the top of the drilling riser and lead it to recycling devices on board the drilling vessel, and further for reuse. Previous practices by letting the drilling liquid into the sea after use is no longer possible because of costs (except when the drilling liquid is sea water), aesthetical considerations and general care for the marine environment.
Handling and use of a 21xe2x80x3 standard marine riser.
The operation and handling of a marine riser is problematic while drilling at large sea depths. Additionally the great volume of drilling fluid in the marine riser requires an extra storage capacity on the drilling vessel, normally 20 m3 per 100 m of riser. At present only a few of fourth- or fifth generation drilling rigs are capable of operating and handling the weight of a 21xe2x80x3 drilling riser at depths between 1000 and 1500 meters of water. These rigs are expensive and cost about 1,300,000 to 1,500,000 NOK/day. By replacing a part of the drilling mud in the 21xe2x80x3 marine drilling riser with air, this will exert buoyancy in the part of the drilling riser which is emptied of mud. The return mud may be sent via a separate 4xe2x80x3-6xe2x80x3 return mud pipe a device (pump) is put in place for artificially lifting the return drilling mud will reduce or eliminate several of the above mentioned limitations. Of one for instance reduces the mud level to a pump-out level 300 m below RKB one may pump out 20 m3/100 m*300 m*2000 kg/m3=120 000 kg=120 tons. Somewhat increased weight due to the mudpump itself and the mud in the return drilling riser must be accounted, but the reduction of weight is considerable.
2. Description of the Related Art
The idea itself, by arranging a separate return mud riser pipe with its own lift pump to return the drilling mud to the drilling vessel, is known as such. U.S. Pat. No. 4,063,602 describes a device for taking out the return mud via a T-pipe connection situated just above the blowout preventer. The purpose is to avoid the fracturing problems in the shallow geological formations when a high column of drilling mud is set up through the height of the riser at great water depths, during the start of the drilling at the seabed, and by relatively shallow drilling depth. From the T-pipe connection the drilling mud may be let out directly into the sea via a valve, and directly out on the seabed. Alternatively the drilling mud may be pumped up through a return pipe to the drilling vessel by means of a pump. The valve from the drilling riser to the T-pipe connection is controlled from the surface.
U.S. Pat. No. 4,063,602 granted in 1977 and U.S. Pat. No. 4,291,772 granted in 1981 both concern separate return riser pipes with pumps arranged near the wellhead valve at the seabed. The state of the art at that time was intended for drilling at far shallower sea depths than what the present invention is arranged for, and the solution with pumps arranged near the well valve at a depth between 1000 and 1500 meters being mentioned in the application as actual implementation depths of the invention, would imply a need for very long supply conductors for energy, and put extreme demands for leak-tightening of the mud return pumps and leak-proofing of pump engines.
U.S. Pat. No. 4,291,772 describes a drilling riser with connection of the return riser pipe at the wellhead, and an application of two separate fluids to maintain the correct mud pressure over the formations is described. One heavy fluid is circulated down via the inside of the drillstring and the return mud level is adjusted to stand in the drilling riser just above the wellhead by means of the return riser pipe and the return lift pump. The level for the heavy return mud in the drilling riser is adjusted by means of the pressure of the lighter fluid standing in the drilling riser. The lighter fluid may be mud, water or air. In order to maintain the pressure in the lighter fluid U.S. Pat. No. 4,291,772 prescribes application of a packer over the lighter fluid and below the kelley. This requires a blowout preventer valve below the kelley. U.S. Pat. No. 4,291,772 thus leads to severe problems when one shall
a) change the diameter of the drillstring,
b) send the drillbit through the blowout preventer and simultaneously maintain the riser margin,
c) set down a casing string.
The different pipes and the drillbit shall firstly be led through the upper blowout preventer valve with a large pressure gradient, and then through the blowout preventer valve by the seabed. That solution becomes unproportionately expensive, difficult to implement and gives a huge time loss by change of drillbit and insertion of casing string.
U.S. Pat. No. 4,291,772 imposes risk of collapse of the drilling riser for the water depths for which the present invention is to be applied for. 21xe2x80x3 drilling risers with 12 mm wall thickness have a collapse depth of about 600 meters water depth. At the time of granting of U.S. Pat. No. 4,291,772 it was hardly actual to drill on more than 600 meters sea depth. If one should base one""s operation on U.S. Pat. No. 4,291,772 while drilling at more than 600 meters of water depth the risk of collapse would be immediate if one should happen to loose the air pressure below the upper blowout preventer valve. This would imply immediate collapse of the drilling riser and loss of the drilling riser and the drillstring. The same arguments are valid against U.S. Pat. No. 4,063,602 which also has the riser lift pump arranged near the seabed and which also has not been thought applied for the sea depths which now are actual for drilling.
Usually drilling risers of 21xe2x80x3 diameter are applied. If the drilling mud level sinks inside the drilling riser below a certain level the water pressure will lead to collapse of the drilling riser at a given depth Dk, depending on the drilling riser""s wall thickness t:                                                                         xe2x80x83                            ⁢              t                                                                          xe2x80x83                            ⁢                              D                k                                                                            --                  --                      --                          --              --                                                                                                  12              ⁢                              xe2x80x83                            ⁢              mm                                                          600              ⁢                              xe2x80x83                            ⁢              m                                                                          16              ⁢                              xe2x80x83                            ⁢              mm                                                          950              ⁢                              xe2x80x83                            ⁢                              m                .                                                        
A collapse of the drilling riser will lead to a risk of complete loss of the drilling mud above the blowout preventer valve. However automatic xe2x80x9cfill-upxe2x80x9d valves exist for letting in seawater into the drilling riser in order to avoid collapse of the drilling riser due to the surrounding pressure.
An emergency disconnection is not mentioned in the above mentioned patents.
If one wishes to avoid blowout, usually the above mentioned riser margin is applied by adjusting up the density of the drilling mud so that the sum of the pressure columns from the remaining drilling mud under the blowout preventer valve and the seawater down to the blowout preventer valve together may resist the pore pressure in all part of the borehole.
xe2x80x83p=p0+xcfx81m2g(hm31 dw)+xcfx81wg(dw)xe2x80x83xe2x80x83(2)
With xcfx81m2 as the new increased density of the drilling mud standing from the bottom of the borehole up to the blowout preventer valve, dw as the water depth, and xcfx81w as the density of sea water.
The present invention concerns a drilling riser for use at great sea depths for drilling by means of a drillstring, of wells in the seabed, with the drilling riser being arranged for connection between a wellhead at the seabed and a vessel, and arranged for use with a drilling fluid with sufficiently high density to balance the fluid pressure from the geological formations, with a sensor arranged to register the level of drilling fluid in the drilling riser, and a return riser pipe with an adjustable mud return riser pipe pump. The new and inventive trait by this drilling riser is that the mud return riser pipe extends from the vessel down to an outlet on the drilling riser at a depth which is substantially below the sea surface, and that the return riser pipe mud pump is arranged near by the outlet and arranged for adjusting the drilling fluid level to a predetermined level near or above the outlet and substantially deeper than the sea surface, and that the drilling fluid has a considerably higher fluid density than what would be sufficient for balancing the fluid pressure from the geological formations by using a drilling fluid column extending all the way up to the sea surface or to the vessel. The invention also concerns a method for establishing a sufficient riser margin in the above mentioned drilling riser. The new and inventive step by the method is that the level of drilling fluid by means of the mud return riser pipe pump is held near or above the outlet, and that the density of the drilling fluid is kept considerably higher than what would be sufficient for balancing or exceeding the fluid pressure from the geological formations by using a drilling fluid column extending all the way up to the sea surface or the vessel, so that the sum of the hydrostatic pressures of the sea water and the remaining drilling mud below the wellhead after either a deliberate or involuntary disconnection of the drilling riser still would balance or be higher than the fluid pressure from the geological formations.
Further traits by the invention will emerge from the dependent claims.
The meaning of xe2x80x9ca depth which is substantially below the sea surface, and at the same time at considerate height above the seabedxe2x80x9d is a depth which preferably is about a hundred meters or deeper below the sea surface, and not as deep as the total water depth down to the seabed, but preferably several hundred meters above the seabed, except from the occasions where the sea depth is so shallow that the mud return riser pipe may be arranged just above the seabed.
Here follows given a closer description of the invention, with references to figure drawings where the invention has been illustrated.